Phase modulation detector



vAug. 19, i947. M. G. CROSBY v PHASE MODULATION DETECTOR Original FiledApril 3, 1943 Y fP/af I Arme/VU.

Patented Aug. 19, 1947 UNITED STATES PATENT OFFICE PHASE MODULATIONDETECTOR Murray G. Crosby, Riverhead, N. Y., assigner to RadioCorporation of America, a corporation of Delaware (Cl. Z50-27) Claims.

My present invention relates to phase modulation detector circuits, andmore particularly to improved forms of a phase modulation detector ofthe piezo-electric crystal type. This application is a division of mycopending Patent No. 2,397,841, dated April 2, 1946.

In the past I have disclosed various circuits for utilizing the inherentproperties of a simple crystal filter to convert phase modulation ofcarrier energy into amplitude modulation for detection. For example, inCommunication by phase modulation, Proceedings of the I. R. E. forFebruary 1939 (pages 126 to 136), I have shown a crystal filter phasemodulation translating network, and have explained the operation thereofon the basis of over and under-neutralization of crystal holdercapacitance.

It is one of the main objects of my present invention to provideimproved and modified types of crystal filter phase modulation (PMhereinafter for brevity) detectors, in each type, or form, of circuitthe basic functioning being considered as involving the application toeach of a pair of opposed rectiers the resultant of crystal-filteredcarrier-energy and modulated carrier energy in normal phase quadraturerelation at resonance.

Another important object of my present invention is to detect PM waveenergy by a process which involves passing the PM energy through apiezo-electric crystal lter to secure substantially unmodulated carrierenergy, applying the unfiltered PM wave energy to a pair of opposedrectiers, shifting the relative phase between the filtered andunfiltered energy to quadrature relation for the resonance condition,and applying the filtered energy to the rectiiiers thereby to cause eachrectifier to rectify its respective Vector resultant energy.

Another object of my invention is to provide a PM wave energy detectionnetwork for PM wave energy or AM (amplitude modulation) wave energy, thenetwork being capable of providing automatic frequency control (AFC)voltage, substantially pure carrier energy for carrier exaltation, andmodulation signal voltage,

Yet another object of my invention is to provide a device forcontrolling the selectivity of a crystal lter circuit feeding opposedrectiers of PM carrier wave energy.

Other objects of my invention are to improve enerally the constructionand operation of PM or AM detectors of the crystal filter type, and moreespecially to provide such detectors in a simple andeconomically-manufacturable form.

Other features will best be understood by reference'to the followingdescription, taken in connection with the drawing, in which I haveindicated diagrammatically several circuit organizations whereby myinvention may be carried into effect.

In the drawing:

Fig. 1 snows one embodiment of the invention;

Fig. la graphically shows an ideal phase shifting characteristic of thecrystal;

Figs. 2a and 2b show vector relations between the filtered andunfiltered signal energy for the unmodulated and modulated statesrespectively;

Fig. 2c vectorially explains the manner of phase detection from anotherviewpoint;

Fig. 2d shows the vector relations between the filtered and unfilteredsignal energy for a condition of off-tune, or oil-resonance; and

Figs. 3 and 3a show two further modifications.

Referring to the accompanying drawing, wherein like reference charactersin the diiferent iigures designate similar circuit elements, there isshown in Fig. 1 one form of a detector circuit which embodies thefeatures of the present invention. The detector network is provided withan input transformer l whose primary circuit 2 is resonated to thecenter, or mean, frequency of the applied modulated carrier wave energy.Since the present invention is not in any way concerned with the sourceof the signal energy, the circuits prior to the primary circuit '2 arenot shown in the drawing. The signals applied to the input transformer Imay be PM, or AM, signal waves. Those skilled in the art are fully awareof the various networks which could be employed prior to circuit 2.Particular reference is made to my aforementioned publication for a morespecific disclosure of such prior networks, where the receiver is of thePM type.

It will be sunicient for the purposes of this application to assume thatthe detector circuit is employed in a PM receiver of the superheterodynetype, and that the carrier frequency of the transmitted PM wave energywill be some predetermined frequency in the high frequency band. Forexample, the band below 25 megacycles (mc.) is particularly desirablefor the radiation of PM signals. In the superheterodyne form ofreception, a local oscillation circuit is employed to reduce the meanfrequency of the received PM carrier energy to an intermediate frequency(I. F.) and it is the PM signal energy at the I. F. mean value which isapplied to the primary circuit 2. Hence, the circuit 2 is resonated tothe I. F. mean value of the incoming PM Wave energy. If desired, anamplitude modulation limiter stage may be employed prior to the circuit2. The modulation signal output of the detector will then be trulyrepresentative of the phase deviations of the carrier, and not of theamplitude variations of the latter.

Considering, now, the detector circuit per se, it is first pointed outthat the circuit comprises a pair of opposed rectifiers 3 and f5. Theserectiers are shown as of the diode type, since such rectiers are simplein construction. However, the present invention is in no way limited tothe particular types of tubes shown, nor, indeed, to the above-mentionedspecific frequencies. The anode of rectifier 3 is connected to one endof the secondary winding I of input transformer I. The last-mentionedconnection includes in series a piezo-electric crystal P and a resistor8.

The crystal P is located between a pair of metallic electrodes in theusual fashion, and the crystal is tuned to the mean frequency of theapplied signal energy. That is to say, the crystal P is tuned to theresonant frequency of the input circuit 2. The electrodes of the crystalmay, if desired, be metal coats suitably provided on the opposite facesof the crystal. The resistor 8 may be shunted by an adjustable condenser8', and the function of the latter will be explained at a later point.The anode of rectifier 4 is connected to the opposite end of thesecondary winding 'I through a path comprising the capacity 9 arrangedin series with resistor IB. Resistor I is shunted by variable condenserI. The right-hand termina1 of condenser 9 is connected by lead I i tothe righthand crystal electrode. Between the midpoint of winding 'i andthe lead II there is connected a resonant circuit which comprises coilI2 shunted by the adjustable condenser i3. Normally, the resonantcircuit I 2-I3 is tuned to the mean frequency value of the applied PMsignal energy. The output load resistors of the circuit are designatedby numerals Ill and i5, and these resistors are connected in seriesbetween the cathodes of diodes 3 and 4, The cathode end of resistor I5is established at ground potential, and each of resistors Id and I isshunted by its respective carrier bypass condenser. The junction ofresistors I4 and I5 is connected by lead I5 to the midpoint of thesecondary winding l.

A second path connects the anode of each rectier to the respective endof winding 1. Thus, condenser 5 connects the anode of diode 3 to theupper end of winding 1, while the condenser 6 connects the anode ofdiode l to the lower end of winding l. Modulation signal energy may betaken off from the cathode end of resistor Id. Furthermore, that samepoint of the output resistor may be tapped for AFC voltage in order tocontrol the frequency of the local oscillator, as is well understood.Where AFC voltage is taken off from the cathode end of resistor Iii, amodulation voltage filter, schematically represented by numeral I'i, isinserted in the AFC output line. Substantially pure, or filtered,carrier energy is taken off from lead I8. The filtered carrier energywill have a frequency equal to the mean value of the applied PM signalenergy. Such filtered carrier energy may be utilized for carrierexaltation detection in the manner disclosed in my U. S. Patent No.2,063,588, granted December 8, 1936. In such case, the pure carrierenergy is fed to a separate phase modulation, or AM, detector. It is notbelieved necessary to show the dernodulator in such case, since thoseskilled in the art will readily understand that it can be a second PMdetector whose input is taken from the input circuit 2 of Fig. l. Ifdesired, the filtering of the energy applied to crystal P may be such asto leave modulation from zero to about 200 cycles on the carrier.

In explaining the functioning of the circuit shown in Fig. 1, it isfirst pointed out that the condenser 9 is adjusted so as to neutralizethe capacity existing between the metal electrodes of crystal P.Referring to Fig. 2a there is shown the vector relations existingbetween the retarded and unretarded voltages insofar as they affect theopposed rectifiers 3 and 4. The PM signal energy which passes throughthe crystal P is stripped of its modulation Side bands, so that there isapplied to the anode of each of rectifiers 3 and l virtually unmodulatedcarrier energy. The vector Ep represents this crystal-filtered carrierenergy applied to the rectifiers. The path from the crystal to therectifier 3 is through resistor 8, while the path from the crystal tothe rectifier 4 is through lead i I and resistor IG. It will, therefore,be seen that the filtered carrier energy at the output electrode ofcrystal P is applied in like polarity, or in parallel, to the anodes ofthe respective diode rectifiers 3 and 4. The passage of the filteredcarrier energy through resistors 8 and I0 to the respective detectorinput electrodes is accomplished without phase shift since the couplingis totally resistive except for the effect produced by condensers 8' andI. The effect of condensers 8 and I0 is compensated for by detuningcircuit IZ-IB as will be described later.

The unltered PM signal energy which passes to each of rectifiers 3 and 4through condensers 5 and 6 respectively is shifted 90 degrees in phaseby virtue of the capacity feed to the respective detector inputelectrodes. Furthermore, since these energies are taken from theopposite ends of the winding 'I they are applied to the opposedrectifiers 3 and 4 in polarity opposition. It will be noted that themidpoint of winding 'i is effectively at ground potential with respectto radio frequencies, because the lead I5 connects the midpoint toground through the condenser in shunt with resistor I5. The vectors E6and E5 denote the unfiltered voltages applied to rectifiers 4 and 3respectively, and it will be seen that these vectors are in phasequadrature with the crystalfiltered carrier energy. This phasequadrature relation of the two voltages at each rectifier results fromthe fact that the unfiltered signal energy is applied to the rectifiers3 and Il by condensers 5 and 6 respectively which are suficiently smallto effect a degree phase shift, and are, also, of substantially equalcapacities so as to produce equal phase shifts both of the unfilteredcarrier and of signal components. The condensers 5 and 6 arenon-selective to phase or frequency variations of the unfilteredcarrier, and, accordingly, permit all signal components to pass to therectifiers 3 and 4. The crystal P, however, effects no phase shift atthe carrier, but substantially removes the phase modulation of thesignal, thereby restoring the carrier substantially to the phase andwave form which it had before modulation at the transmitter. The crystalP is, of course, selective for frequencies off resonance. This followsfrom the sharp selectivity characteristic of the crystal, as depicted inFig. la.

The vector representing the resultant signal energy at each rectifier isalso indicated in Fig. 2a. Thus, theV vector En represents the resultantenergy applied to rectifier 3. The vector En represents the resultantenergy applied to rectifier d. Fig. 2a depicts the situation when themean frequency value of the applied PM signal energy is instantaneouslyequa-l to the frequency of circuit 2 and the frequency of the crystal P.The rectiiied outputs of each rectifier will, therefore, be equal, andthe effective voltage at the cathode end of resistor Iii will,therefore, be Zero. In other words, for the in tune state no AFC bias isdeveloped.

In explaining the operation of this circuit, there are two separateconditions which must be considered. One is the case of thedernodulation of a phase modulated signal, and the second is the case ofthe detection of slow frequency variations to obtain AFC potentials.These two cases represent two different degrees of modulation that areacted upon by the crystal iilter in different manners. For the case ofthe relatively rapid modulation represented by the phase modulations ofthe signal, the filter acts as a devic which selects the carrier fromthe side bands, and provides the equivalent of a synchronized localcarrier free of modulation. When an uninodu lated carrier is receivedthere delivered to each of diode rectiers 3 and il voltages, one afiltered carrier from crystal l: without phase change, and the other anunfiltered carrier substantially 90 degrees different in phase from theltered carrier. When the received carrier is phase-modulated theiiltered carrier remains as before, but the unfiltered signal energy issupplied to the rectiiiers 3 and il in phases differing from the 90degree, or quadrature, relation to an extent determined by the degree ofphase modulation. If the degree of phase modulation is small, arelatively small direct current voltage is built up across rectifieroutput resistors and l5 due to the signal voltage increasing on one ofthe rectiers and decreasing on the other. The greater the degree ofphase modulation the greater the combined voltage of the unfilteredsignal energy and the filtered carrier from crystal l; on of the and theless the s un of such voltages on the other rectifier. rhe polarity ofthe direct current voltage drop across the load resistors lil and l5 ofthe opposed rectiers depends on the direction of the phase change of thereceived signal energy.

For the case of the relatively slow variations in frequency of theincoming signal, the crystal filter acts as a retard circuit havino` anoutput phase which varies with the frequency of the input. For thiscase, the circuit acts like a very narrow-band frequency modulationdiscriminator. The solid curve in TEig. la, shows frequency vs. phaseshift characteristic of crystal P. At Fc, the center frequency, thecrystal provides zero phase shift.

The vector diagrams of Figs. 2a, 2b and 2c show the conditions for thecase of phase modulation detection. The filtered carrier, which isrepresented by vector Ep, remains fixed in phase. The unfilteredmodulated represented by vectors E5 and EG, vary in phase to producedifferentially modulated resultants Eo and En' which are fe to theopposed detectors. The manner in which the modulated signal varies inphase with respect to the ltered carrier is shown in l'iig. 2c. Acondition of modulation in one direction is shown in Fig. 2b. Theunmodulated condition is shown in Fig. 2a.

The vector diagrams of Figs. 2a and 2d show the conditions for the caseof AFC detection. Fig. 2a shows the in-tune condition which is effectedwhen the applied signal carrier frequency is in the l f middle' of thecrystal lter characteristic. The

diagram of Fig. 2d shows the relations for an oif-tune condition. Itwill be noted that the carrier (or crystal output) phase shifts for theolf-tune condition. This phase shift is brought about by the phasecharacteristic of the crystal, which is similar to that of an ordinaryresonance circuit, as shown in Fig. la. The magnitude and sense of phaseshift of the ltered carrier energy are respectively dependent on theamount and direction of frequency departure of the modulated carrierenergy at circuit 2 relative to the predetermined frequency Fc. Thelatter is, of course, the resonant frequency of crystal P. The signalenergy passing through condensers 5 and t will not shift in phase inresponse to carrier frequency departures from Fc. This follows from thefact that condensers 5 and e are non-selective. elements. Hence, and asshown in Fig. 2d, the resultant vector voltages En and Eb will Vary inrelative magnitude depending on the extent and sense of the aforesaidfrequency departure. rhese relatives variations in Eo and E0 aretranslated into corresponding direct current voltage variations acrossload resistors it and l5, and the differential of these direct currentvoltages is used as AFC bias after iiltering at il. It is seen that thecarrier phase for the off-tune condition is no longer in its properquadrature re iationship with the uniiltered signal so that it might bethought that the detection of phase modulation would be impaired.However, this olf-tune condition is never allowed to exist to anyappreciable degree, since the AFC circuit functions to correct thetuning and maintain it in the in-tune condition represented by Fig 2a.

Winding l' has its midpoint grounded, as pointed out above, and parallelresonant circuit l2, I3 connects the grounded midpoint of coil 'I to thelead l l which connects the output electrode of the crystal P to therectiiiers. The circuit l2, I3 is, accordingly, in effect connectedbetween the output side of crystal P and ground. Circuit I2, i3 may .betuned to crystal frequency, or may be detuned relatively thereto, andacts as a coupling circuit of finite impedance between the output ofcrystal P and the rectifiers 3 and It. The circuit l2, i3 increases theQ of the crystal beyond what it would be if the resistance of circuitIE, i3 were innite. By detuning resonant circuit l2, i3 the phase of theltered carrier energy can be shifted to a predetermined extent. Thisphase shift can be compensated by each shunt capacity 8 and lil. Each ofresistors 8 and l is, therefore, shunted by a respective compensationcondenser. In this way, the selectivity of the crystal P may be improvedwithout af.. footing the zero phase shift state of the ltered carrierenergy. In other words, detuning the circuit l2, i3 not only will causea phase shift of the filtered carrier, but will also act to increase theselectivity of the crystal filter. If desired, resistor 8 and condenser8 may be interchanged with condenser 5, and resistor lll and condenserlll may be interchanged with condenser l. This would in no way affectthe relative normal phase quadrature relation between the retarded andunretarded PM signal energy as depicted in Fig. 2a.

In the arrangement of Fig. 3 the filtered carrier energy is derived fromthe primary resonant circuit 2. lThe unltered signal energy is shifteddegrees by virtue of the magnetic coupling M existing between thewindings of the input transformer l. In this case, the crystal P isconnected between the upper end of the primary winding' of the inputtransformer and the midpoint of the secondary winding. The condenser 9,connected between the midpoint of the secondary Winding and the lowerend of the primary winding, functions to neutralize the crystalinter-e1ectrode capacity. The pure carrier energy is taken from theoutput electrode Vof the crystal. Otherwise, the circuit functions inthe same manner as described in connection with Fig. l. It is sufficientto point out in connection with this modification that the filtered PMsignal energy is applied to the anodes of rectifiers 3 and 4 in likepolarity by virtue of the connection of the crystal to the midpoint ofthe secondary Winding. On the other hand, the unfiltered signal energyis first shifted 90 degrees in phase by the magnetic coupling M, and thephase shifted energies are applied in polarity opposition to the anodesof the opposed rectiiiers. It may happen that the circuit shown in Fig.3 will give rise to second harmonies in the pure carrier energy takenoff from the crystal P. This is brought about by full-wave rectifiedvoltage, rectified from the secondary of transformer I and appearingacross circuit I2, I3.

In that case, the rectiiiers 3 and 4 are reversed in connections toeliminate the full-Wave connection, and cause the rectifiers to conductsimultaneously instead of alternately and thereby suppress theproduction of second harmonics. This is done by connecting the loadresistors I4 and I5 in the manner shown in Fig. 3a. The anode ofrectifier 3 is connected to the junction of resistors I4 and I5, whilethe cathode of diode 3 is connected to the upper end of resistor I4. Inother words, the only change that need be made in the circuit of Fig. 3is that indicated with respect to resistor I 4 and its associated diode3. The diode-resistor condenser is no longer across resistor I4, buttakes the form of condenser 3 inserted from the cathode of diode 3 tothe connection leading to the upper end of the secondary winding of theinput transformer.

While I have indicated and described several systems for carrying myinvention into eect, it will be apparent to one skilled in the art thatmy invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention.

What I claim is:

l. In a detector of phase modulated signalling energy, a signal inputtransformer having a primary resonant circuit tuned to the meanfrequency of applied signal energy, a secondary'circuit tuned to saidmean frequency, a piezo-electric crystal element tuned to the said meanfrequency, means connecting saidv crystal element between one side ofthe primary circuit and the mid-point of the secondary circuit, a firstrectier, means connecting the rectifier electrodes in circuit with saidcrystal element thereby to have the crystal outputenergyapplied-thereto, a second rectifier in circuit with the crystal elementhaving said output energy applied thereto in like polarity, a commonoutput circuit connecting said rectifiers, separate connections fromrespectively separated points of the input transformer to th'erespective rectifiers for applying thereto unltered phase modulatedenergy, said separated points being pointsof opposite polarity on saidsecondary circuit, a resonant circuit, normally tuned to said meanfrequency, in circuit withI the crystal element, said resonant. circuitincluding means for adjusting its frequency whereby the phase relationbetween the crystal output energy and the unfiltered energy may bevaried, and means for utilizing the crystal output energy forcarrier-exalted demodulation.

2. In a frequency-sensitive system, a first resonant circuit adapted tohave frequency-variable signals applied thereto, means establishing apoint of the circuit as a reference potential point, a second resonantcircuit coupled to the first circuit, said circuits being tuned to adesired frequency, a first rectifier circuit connected from one side ofsaid second resonant circuit to an intermediate point thereof, a secondrectifier circuit connected from the opposite side of the secondresonant circuit to said last named intermediate point, a piezo-electriccrystal, tuned to said desired frequency, connected from a point on saidfirst resonant circuit on one side of said reference potential point tosaid intermediate point, a third resonant circuit, tuned tosubstantially said desired frequency, included in both of said rectifiercircuits, said third circuit having an input terminal connected to saidintermediate point, and means adapted to derive from said rectifiercircuits the differential resultant of the rectified signal voltagesthereof.

3. In a frequency-sensitive system, a rst resonant circuit adapted tohave frequency-variable signals applied thereto, means establishing apoint of the circuit as a reference potential point, a second resonantcircuit coupled to the first circuit, said circuits being tuned to adesired frequency, a first rectifier circuit connected from one side ofsaid second resonant circuit to an intermediate point thereof, a secondrectier circuit connected from the opposite side of the second resonantcircuit to said last named intermediate point, a piezo-electric crystal,tuned to said desired frequency, connected from a point on said firstresonant circuit on one side of said reference potential point to saidintermediate point, a third resonant circuit, tuned to substantiallysaid desired frequency, included in both of said rectifier circuits,said third circuit having an input terminal connected to saidintermediate point, and means adapted to derive from said rectifiercircuits the differential resultant 0f the rectified signal voltagesthereof, means for neutralizing the inter-electrode capacity of saidcrystal comprising a condenser connected from a point on said firstresonant circuit on the opposite side of said reference potential pointto said intermeditae point.

4. In a frequency-sensitive system, a first resonant circuit adapted tohave frequency-variable signals applied thereto, means establishing apoint of the circuit as a reference potential point, a second resonantcircuit coupled to the rst circuit, said circuits being tuned to adesired frequency, a first rectifier circuit connected from one side ofsaid second resonant circuit to an intermediate point thereof, a secondrectifier circuit connected from the opposite side of the secondresonant circuit to said last named intermediate point, a piezo-electriccrystal, tuned to said desired frequency, connected from a point on saidfirst resonant circuit on one side of said reference potential point tosaid intermediate point, a third resonant circuit, tuned tosubstantially said desired frequency, included in both of said rectifiercircuits, said third circuit having an input terminal connected to saidintermediate point,

9 and means adapted to derive from said rectier circuits the diierentialresultant of the rectied signal voltages thereof, a signal amplifiertube preceding said rst resonant circuit, means connecting th'e tubeoutput electrode to said point on said rst resonant circuit to whichsaid crystal is connected, and means connected to said refer- 10 encepotential point adapted to apply a positive direct current voltage tosaid output electrode.

5. In a system as defined in claim 2, said rectier circuits includingrespective diodes, one of the diodes being directly shunted across itsrespective load resistor.

MURRAY G. CROSBY.

